Hi wang,
well good question i must say, but u forgot one thing man and that is bandwidth.You cannot allocate the same bandwidth for voice and data, even if u do that it would not be logical coz voice is delay sensitive whereas data is not. Even if ur data packets get misordered they can be collected at remote end and re ordered so that u never come to know whats happening, whereas if u do this to voice, u will speak good morning from ur end and will recieve the response after some minutes from the other end, which i suppose, u would never like to have. Thats why we treat voice and data separately inspite of packetizing both of them
hope that serves the purpose.
cya,
vivek